1. Field of the Invention
The present invention pertains to an optical prism that is combined with multiple image sensing elements employed in an image sensing device such as a camera or scanner.
2. Description of the Related Art
Image sensing devices in which an optical prism is combined with multiple image sensing elements have conventionally been proposed. In recent years, because it is advantageous in reducing both harmful light and the number of components, as well as in achieving compactness and light weight, part of the housings of the image sensing elements are glued to the optical prism on its image forming light exit surfaces using a resin, etc., that hardens under UV light, such that the image sensing elements and the optical prism are combined.
FIG. 5 is a rough sketch of an area where an image sensing element is attached in a conventional image sensing device. 10 is an optical prism, and 4 is a housing containing an image sensing element. Housing 4 is connected to surface 10a from which image forming light rays exit toward the image sensing element. A method in which the length of the optical path regarding the image sensing element and the optical prism is adjusted while the image sensing element is combined with the optical prism is disclosed in Japanese Laid-Open Patent Hei 5-232305.
FIG. 6 is a side elevational view of the important parts of an image sensing device using this method to explain the method to adjust the length of the optical path. The optical prism comprises first glass member 11, second glass member 12 and third glass member 13, and the length of the optical path of the light rays that reach three image sensing elements is adjusted by adjusting the relative positions of these three glass members. 14 is an image sensing lens that is attached to the optical prism by means of attaching member 15.
In order to glue the housing, a resin that hardens under UV light is sometimes used because of its ease of use and because work time may be freely set. To harden the resin and affix the image sensing element, it is necessary to irradiate UV light on the adhesion area. In the configuration shown in FIG. 6 in which an image sensing lens is combined with an optical prism, it would be difficult for UV light to be irradiated via the incident surface of the optical prism. Where UV light is to be irradiated onto the adhesion area in the configuration shown in FIG. 6, the UV light must be irradiated using a surface other than the surface from which the image forming light rays exit.
As a method to irradiate UV rays using a surface other than the image forming light exit surface, a light guiding member is sometimes added in the vicinity of the adhesion area for the purpose of irradiation of UV light. FIG. 7 is a side elevational view of an optical system to explain one conventional example of this method. 7 is a light guiding member that is affixed beforehand to adhesion surface 4a of the housing. When housing 4 is affixed to optical prism 10, UV light 6 is irradiated via one edge of light guiding member 7, causing the UV light-hardened resin to harden.
When UV light is irradiated under the method shown in FIG. 7, the following problems arise due to the contraction of the resin during the hardening process.
Where UV light is not irradiated evenly over the entire adhesion surface, the hardening of the resin does not proceed evenly and the correct positioning is lost during the hardening process, and due to residual stress, separation or displacement may occur. In addition, where the amount of UV light irradiated on the center part is insufficient, hardening will proceed from the edge to the center part, and as a result there is a danger that boundary surfaces will remain in the vicinity of the center part and that cracking may occur in some cases.
Further, in an image sensing device in which multiple image sensing elements are employed, in order to affix the image sensing elements in positions that are optically equivalent or appropriate in terms of design, positioning along a total of six axes, i.e., positioning along three axes comprising a three-dimensional coordinate system and in terms of the rotation around said three axes, is required.
However, because only changes in the directions along the exit surface is permitted under the method shown in FIG. 5, neither focus adjustment in which the image sensing element is moved in the directions along the optical axis nor angling adjustment in which the image sensing element is tilted relative to the exit surface may be performed. As a result, the image sensing device suffers from image quality deterioration, such as a reduction in resolution or image distortion.
In the conventional example shown in FIG. 6, it is difficult to make complete adjustment along all six axes, and it is unavoidable that the positioning error pertaining to the image sensing elements and the housing will negatively affect the image quality.
Although this negative influence may be mitigated by improving the precision in the combination of the image sensing elements and the housing, among the tilting of the image formation plane and misplaced focus positions pertaining to light rays that should form an image on each of the image sensing elements, those problems caused by the image sensing lens cannot be corrected.
Therefore, in an image sensing device in which a dedicated image sensing lens and an optical prism are affixed to each other, in consideration of the reduction in image quality that occurs due to improper positioning of the image sensing elements and the optical prism, the performance of the image sensing lens must be improved, which increases its size, weight and cost.